Method and apparatus for optimizing signal transformation in a frame-based communications network

ABSTRACT

A method of and apparatus for optimizing signal transformation from a twisted pair transmission line to a combination transmitter and receiver for a frame-based communications network, the transmitter having a transmit output pair port for transmitting signals onto the frame-based communications network over the twisted pair transmission line, the receiver having a receive input pair port for receiving signals from the frame-based communications network over the twisted pair transmission line. A transformer is coupled between the twisted pair transmission line and each of the transmit output pair port and the receive input pair port. The transformer has a coil across the twisted pair, a transmit coil across the transmit output pair port, and a receive coil across the receive input pair port. A transfer ratio between the transmit coil and the coil across the twisted pair is optimized for transmitting signals. A transfer ratio between the receive coil and the coil across the twisted pair is optimized for receiving signals.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This patent application claims the benefit of the filing date ofU.S. Provisional Patent Application No. 60/197,224 filed Apr. 14, 2000;and U.S. Provisional Patent Application No. 60/196,002 filed Apr. 7,2000; the entire contents of both of which are hereby expresslyincorporated by reference.

[0002] This patent application is further related to the following U.S.Patent Applications filed concurrently herewith and commonly assigned,entitled “A Method of Sharing Information among a Plurality of Stationsin a Frame-based Communications Network”, “A Method of Enhancing NetworkTransmission on a Priority-enabled Frame-based Communications Network”,“A Method of Determining a Start of a Transmitted Frame in a Frame-basedCommunications Network”, “A Method of Determining an End of aTransmitted Frame in a Frame-based Communications Network”, “A Methodfor Providing Dynamic Adjustment of Frame Encoding Parameters in aFrame-based Communications Network”, “A Method for Selecting FrameEncoding Parameters in a Frame-based Communications Network”, “A Methodfor Selecting Frame Encoding Parameters to Improve TransmissionPerformance in a Frame-based Communications Network”, “A Method ofDetermining a Collision Between a Plurality of Transmitting Stations ina Frame-based Communications Network”, “A Method of ProvidingSynchronous Transport of Packets Between Asynchronous Network Nodes in aFrame-based Communications Network”, “A Method of Controlling DataSampling Clocking of Asynchronous Network Nodes in a Frame-basedCommunications Network”, “A Method for Distributing Sets of CollisionResolution Parameters in a Frame-based Communications Network”, “AMethod and Apparatus for Transceiver Noise Reduction in a Frame-basedCommunications Network”, “A Method for Selecting an Operating Mode for aFrame-based Communications Network”, and “A Transceiver Method andSignal Therefor Embodied in a Carrier Wave for a Frame-basedCommunications Network”.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to the field of communications,and, in particular, to a frame-based communications network.

[0004] As computers become more and more cost effective for the everydayconsumer and for small businesses, such computers become more plentifulfor use within local area environments such as homes, office buildingsand the like. For example, within a home a person with a computer in thebedroom, and another in the living room, may want to share common files,utilize a common digital subscriber line (DSL), or otherwise transferinformation between the computers. Accordingly, various technologies arebeing developed for computer interconnection of multiple computerslocated within such environments. One example of such technologies arethe Home Phoneline Network Alliance (HPNA) specifications for local areanetwork (LAN) computer interconnection which utilize existing telephonelines within the local environment for the transmission of data packetsbetween the computers.

[0005] FIG 1 a shows in block diagram form a general home networkingenvironment within which the present invention can be implemented. Homenetwork 10 includes existing (installed) plain old telephone service(POTS) wiring 12, network clients 14, the computer port side of modem 16and fax 18. POTS wiring 12 provides wiring infrastructure used tonetwork multiple clients at a customer premises (e.g., home) 20. POTSwiring 12 can be conventional unshielded twisted pair (UTP) wiring thatis generally routed internally in the walls of the customer premises 20to various locations (e.g., rooms) within the customer premises.Subscriber loop 22 (also called a “local loop”) is a physical wiringlink that directly connects an individual customer premises 20 to theCentral Office through telephone network interface 24, a demarcationpoint between the inside and outside of customer premises 20. Ofparticular importance for residential networks are systems that providecommunication between computers as reliably and with as high a data rateas possible. Communication over residential telephone wiring is providedthrough inventive frame-oriented link, media access and physical layerprotocols and implementation techniques associated therewith describedherein.

[0006] Referring now to FIG 1 b, those skilled in the art can appreciatethat home phone-line network configuration 10 can also utilize interface6010 to provide signals outside customer premises 20. For example,interface 6010 can include a V.90 modem as described above, connectedthrough the central office to an internet service provider. Interface6010 can include an ADSL modem, a VDSL modem or the like transportinterface.

[0007] Another desired solution for high speed data communicationsappears to be cable modem systems. Cable modems are capable of providingdata rates as high as 56 Mbps, and is thus suitable for high speed filetransfer. In a cable modem system, a headend or cable modem terminationsystem (CMTS) is typically located at a cable company facility andfunctions as a modem which services a large number subscribers. Eachsubscriber has a cable modem (CM). Thus, the CMTS facilitatesbidirectional communication with any desired one of the plurality ofCMs. Referring to FIG. 1c, a hybrid fiber coaxial (HFC) network 1010facilitates the transmission of data between a headend 1012, whichincludes at least one CMTS, and a plurality of homes 1014, each of whichcontains a CM. Such HFC networks are commonly utilized by cableproviders to provide Internet access, cable television, pay-per-view andthe like to subscribers. Approximately 500 homes 1014 are in electricalcommunication with each node 1016, 1034 of the HFC network 1010,typically via coaxial cable 1029, 1030, 1031. Amplifiers 1015 facilitatethe electrical connection of the more distant homes 1014 to the nodes1016, 1034 by boosting the electrical signals so as to desirably enhancethe signal-to-noise ratio of such communications and by thentransmitting the electrical signals over coaxial conductors 1030, 1031.Coaxial conductors 1029 electrically interconnect the homes 1014 withthe coaxial conductors 1030, 1031, which extend between amplifiers 1015and nodes 1016, 1034. Each node 1016, 1034 is electrically connected toa hub 1022, 1024, typically via an optical fiber 1028, 1032. The hubs1022, 1024 are in communication with the headend 1012, via optical fiber1020, 1026. Each hub is typically capable of facilitating communicationwith approximately 20,000 homes 1014. The optical fiber 1020, 1026extending intermediate the headend 1012 and each hub 1022, 1024 definesa fiber ring which is typically capable of facilitating communicationbetween approximately 100,000 homes 1014 and the headend 1012. Theheadend 1012 may include video servers, satellite receivers, videomodulators, telephone switches and/or Internet routers 1018, as well asthe CMTS. The headend 1012 communicates via transmission line 1013,which may be a T1 or T2 line, with the Internet, other headends and/orany other desired device(s) or network.

[0008] Given the HPNA environment and the Cable Modem Systemenvironment, an opportunity exists for a system provider to integrateeach respective environment with voice services. FIG 1 d depicts such anintegrated environment. As can be seen in FIG 1 d, a connection point inthe home to the telephony world (e.g., the world of video, voice, highspeed data network traffic), could be provided to a home user throughcable modem 1046 which would include an HPNA transceiver. The cablemodem system provider may also wish to accomodate providing telephoneservice along with high speed data service. A home computer user, ratherthan using a traditional modem to connect to an internet serviceprovider, would find it convenient to utilize cable modem 1046, takingadvantage of the very high speed data service provided by the cablemodem. Having a cable modem customer, the cable modem provider may alsofind it commercially beneficial to offer video feeds, and telephoneservice over the same cable modem network.

[0009] A cable modem having an HPNA V2 transceiver included therein, canreadily interface into the home phone line network through the telephonejack within the home. Computers coupled to the home network thencommunicate through the cable modem to the outside telephony world asdescribed above. Telephone service coming from outside the customerpremises over the cable modem system would be in a digitized packetizedformat. It would then proceed over the HPNA network in the samedigitized packeting format. If the user, in addition to having computersand the like attached to the HPNA network, wished to have an analogtelephone(s) connected to the HPNA, the telephone'(s) analog signalwould go through a digital conversion and put the digital informationinto packets for passing the packets back and forth over the network.The analog telephone signal is sampled and packetized at the appropriateclock rate creating the packet after a certain number of samples.

[0010] Therefore, to effectively operate in such communications networkenvironments a need exists for a method and apparatus for optimizingsignal transformation in a frame-based communications network. Thepresent invention as described and claimed in this application providesa solution to meet such need.

SUMMARY OF THE INVENTION

[0011] In accordance with the present invention, a method of andapparatus for optimizing signal transformation from a twisted pairtransmission line to a combination transmitter and receiver for aframe-based communications network is provided, the transmitter having atransmit output pair port for transmitting signals onto the frame-basedcommunications network over the twisted pair transmission line, thereceiver having a receive input pair port for receiving signals from theframe-based communications network over the twisted pair transmissionline. A transformer is coupled between the twisted pair transmissionline and each of the transmit output pair port and the receive inputpair port. The transformer has a coil across the twisted pair, atransmit coil across the transmit output pair port, and a receive coilacross the receive input pair port. A transfer ratio between thetransmit coil and the coil across the twisted pair is optimized fortransmitting signals. A transfer ratio between the receive coil and thecoil across the twisted pair is optimized for receiving signals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIGS. 1a, 1 b, 1 c and 1 d are simplified block diagrams showing ahome networking environment within which the present invention can beimplemented.

[0013]FIG. 2 is a seven-layer network stack model, according to the ISOseven-layer network standard, as used in accordance with the presentinvention.

[0014]FIGS. 3a and 3 b show a broadcast/multipoint network and apoint-to-point network, respectively, for use in accordance with thepresent invention.

[0015]FIGS. 4a and 4 b show respectively an integrated MAC/PHY aspectand an analog front end aspect of an embodiment of the presentinvention.

[0016]FIGS. 5a and 5 b depict the metallic power spectral densityassociated with the transmitter in accordance with the presentinvention.

[0017]FIG. 6 shows the magnitude of the transmitter output in accordancewith the present invention.

[0018]FIGS. 7 and 8 depict maximum peak-to-peak interferer level overfrequency range in accordance with the present invention.

[0019]FIG. 9 shows minimum impedance over frequency range.

[0020]FIG. 10 shows an example of input impedance in view of a lowerbound mask over frequency range in accordance with the presentinvention.

[0021]FIG. 11 shows in functional block diagram form an embodiment of atransceiver in accordance with the present invention.

[0022]FIG. 12 depicts a split winding transformer in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] A communications network typically includes a group of nodesinterconnected by a transmission medium. The term “node” relates to anydevice that shares frames of data with other nodes in the network.Devices that may make up a node are computers, printers, scanners, etc.A node may also be a telephone, a television, a set-top box fortelevisions, a camera or other electronic sensing or communicationdevice. Any device that can send and/or receive frames of data withother devices via a communication medium may be a node for purposes ofthe present invention.

[0024] The transmission medium that links each node in a network isequally one of a diverse family of media. Common media used includeunshielded twisted pair (e.g. phone wire, CAT-5 cabling), power lines,optical fiber, coaxial cable and wireless transmission media. Theoperations that each individual node performs in order to access datafrom, and transmit data to, the rest of the network may be logicallybroken down into seven layers according to the ISO Open SystemsInterconnection (OSI) seven-layer network model, which is also referredto as the “network stack”. The seven layers, from the bottom to the topare: 1) the PHYSICAL layer, 2) the DATA LINK layer, 3) the NETWORKlayer, 4) the TRANSPORT layer, 5) the SESSION layer, 6) the PRESENTATIONlayer, and 7) the APPLICATION layer. FIG. 2 illustrates the ISOseven-layer reference model.

[0025] The PHYSICAL layer, or physical link layer, or PHY, is concernedwith transmission of unstructured bit stream traffic over physicalmedia, and relates to the mechanical, electrical, functional, andprocedural characteristics to access and receive data from the physicalmedium. The DATA layer, sometimes referred to as the data link layer,provides for the reliable transfer of information across the physicallink. It is concerned with sending frames, or blocks of data, with thenecessary synchronization, error control, and flow control. The NETWORKlayer separates the uppermost layers from the transmission and switchingtechnologies used to connect nodes. It relates to establishing,maintaining, or terminating connection between nodes.

[0026] The TRANSPORT layer relates to reliability and transparency indata transfers between nodes, and provides end-to-end error recovery andflow control. The SESSION layer provides control to communicationsbetween applications, and establishes, manages, and terminatesconnections between cooperating applications. The PRESENTATION layerprovides independence to the application processes from differences indata syntax or protocols. Finally, the highest layer, the APPLICATIONlayer, provides access to the OSI environment for users. Much more hasbeen written about the benefits and distributed functionality of such anarrangement of layers and need not be recounted here.

[0027] In frame-based networks, there are two fundamental models ortopologies: 1) broadcast/multipoint networks, where all nodes arephysically attached to the same network medium, and use a single, sharedchannel and frames transmitted on the network are visible to all nodes;and 2) point-to-point networks, where pairs of nodes are connected toeach other with communication channels which are not connected to anyother nodes on the network. Frames transmitted on one channel are notvisible to nodes on other channels unless the frames are retransmittedonto the other channels by a node that is connected to multiplechannels. Each channel may use a separate segment of the network medium,or multiple channels may share a single segment using e.g., FrequencyDivision Multiplexing or Time Division Multiplexing techniques. Onecommon example of such a point-to-point network topology is that usedfor IEEE 10BaseT 802.3 networks, with network nodes connected viapoint-to-point Category 5 unshielded twisted pair cable, usingmulti-port devices called hubs to retransmit frames received from onenetwork segment to all other segments.

[0028]FIGS. 3a and 3 b show a broadcast/multipoint network and apoint-to-point network, respectively, for use with the presentinvention. In FIG. 3a, representative nodes 140 a, 140 b, 140 c arecommunicatively coupled with a common transmission medium 250 throughindividual segments 240 a, 240 b, 240 c respectively. Thus, a messagecontaining a broadcast destination address sent from one node is sent toall other nodes coupled with transmission medium 250. In FIG. 3b, nodes140 d, 140 e, 140 f are communicatively coupled to each other byindividual segments 260 d, 260 e, 260 f respectively of transmissionmedia and hub 255. Messages sent from one node to another node on onesegment are not visible to nodes on other segments unless they areretransmitted by a node that is connected to multiple segments, such ashub 255 in a network. Segments 240 a, 240 b, 240 c and commontransmission medium 250 may be (but are not restricted to) a phone line,a power line, a wireless medium, coaxial cable, or a fiber optic medium.Reference to FIGS. 3a and 3 b should be made with respect to thedescription of the embodiments of the invention as set forth below.

[0029] Each node in either type of network has within it a device thatpermits the node to send and receive data frames in the form ofelectrical, electromagnetic, or optical signals. The device isconventionally a semiconductor device implementing the PHYSICAL layer ofthe network connectivity, and the medium access control (MAC) portion ofthe DATA layer of network connectivity.

[0030] Returning to FIG. 2, there is shown a basic network illustratinga network communication protocol between first node 102 that runs anapplication (“APP X”) and another node 104 that runs the same ordifferent application (“APP Y”). Nodes 102 and 104 communicate message108 via transmission medium 106. In the example shown in FIG. 2, whennode 102 has message 108 to send to node 104, it transfers the messagedown through its network stack on the left, from layer to layer.Application header (AH) 103 is appended to message 108 in theAPPLICATION layer, to identify the application being executed by node102. Original message 108, plus the application header AH, is passed tothe PRESENTATION layer, where it is again appended with a presentationlayer header (PH) 105. Such process continues, accordingly addingsession header (SH) 107, transport header (TH) 109 and network header(NH) 111 down to the DATA layer, where the message and appended headersis encapsulated with data layer header (DH) 112 and start of frame (SOF)indicator 113. The DATA layer also may add data trailer (DT)114 and endof frame (EOF) indicator 115. Data layer header 112 may include a sourceaddress (SA) to identify node 102 sending the message, and may alsoinclude a destination address (DA) to identify the intended recipient orgroup of recipients.

[0031] The message with appended headers, trailers and indicators isthen passed to the PHYSICAL layer where it is passed on to networktransmission medium 106. When received by node 104, the reverse processoccurs in the network stack of node 104. At each layer, the headerand/or trailer information is stripped off as message 108 ascends thenetwork stack.

[0032] The details of the network stack in FIG. 2 are provided forreference only, and the present invention is not limited to functioningwith network stack implementations that exactly match FIG. 2.

[0033] Referring still to FIG. 2, the lower two layers are described infurther detail. It should be understood that these layers are typicallyimplemented as a combination of logic and memory storage that isconfigured to carry out the task of the layer. The logic can be in theform of hardware, software, firmware, or a combination of those. Eachlayer may also be implemented using programmable gate array (PGA)technology, such as system programmable gate arrays (SPGA) and fieldprogrammable gate arrays (FPGA). Also, each layer, or a combination ofthe layers, may be implemented as an integrated circuit or softwareprogram. Therefore, it should be apparent to those skilled in the art,that there are many ways in which to implement the inventions describedherein.

[0034]FIG. 2 shows DATA layers 120 a, 120 b and PHYSICAL layers 220 a,220 b for a representative pair of nodes 140 a, 140 b according to theinvention. Each node has within it semiconductor device(s) thatimplement the PHYSICAL layer as well as the medium access control (MAC)and Link Layer portions of the DATA layer, such as that implemented bythe Broadcom Corporation Model BCM 4210 Controller. As discussed above,the PHYSICAL layer is concerned with transmission and reception of bitstream traffic to and from the transmission medium. Transmitters andreceivers, described in more detail below, form a transmission mediuminterface, and may be implemented as a single device or separatedevices.

[0035] Referring now to FIGS. 4a and 4 b, an embodiment implementing theinventive concepts is depicted wherein, for example, a device such ascomputer 14 can be interconnected therethough to premises UTP wiring asset forth in FIG. 1a, and through which the protocol set forth in FIG. 2is processed. FIG. 4a shows in block diagram form the controller aspectsof the embodiment, while FIG. 4b show typical network interface device(NID) analog front end aspects of the embodiment.

[0036] Referring to FIG. 4a, controller 300 is a fully integratedMAC/PHY device that transmits and receives data (e.g., 10 Mbps and aboveas implemented by the aforementioned Broadcom Corporation Model BCM4210, 4211, 4413 controllers). Controller 300 includes bus interface310, such as a PCI or MSI bus interface for communication in accordancewith well-known PC-based and/or peripheral/internet appliancearchitectures. Controller 300 also includes digital PHY 320 having aFDQAM/QAM transmitter and receiver interfacing with the analog front endand MAC 330, coupling to bus interface 310 through transmit (TX) FIFO340 and receive (RX) FIFO 350. Bus interface 310 also has the capabilityof similarly communicating with other devices 360, such as a v.90 modemthrough v.90 modem interface or a 10/100 Fast Ethernet bus through a10/100 Fast Ethernet interface, and their respective transmit (TX) FIFO370 and receive (RX) FIFO 380. The operations of such bus interfaces andTX/RX FIFOs are well known in the art and are not described in moredetail. The operation of the MAC/PHY aspects of the embodiment aredescribed in more detail herein below.

[0037] Referring to FIG. 4b, NID analog front end 400 connectscontroller 300 depicted in FIG. 4a to a transmission medium 106 such asa premises UTP wiring as depicted in FIGS. 1a, 1 b and 1 c. Analog frontend 400 includes digital input/output (I/O) circuit 410 for transferringsamples and is coupled to a transmit path and a receive path. DigitalI/O 410 includes clock 412 for driving controller 300 with a 64 MHz+/−100 ppm clock generated by 64 Mhz crystal 414. The transmit pathincludes digital-to-analog converter 420 for converting 10 bit sampledata to an analog signal, automatic gain controller 425 for settinggains based upon input received by digital I/O 410, filter 430,transmit-off switch 435, and is coupled to phoneline connector 450, suchas a UTP wiring RJ11 connector, through electronic hybrid 440 forbuffering signals and filter/transformer/electronic protection circuit445. The receive path includes analog-to-digital converter 460 forsending valid sample data, variable gain amplifier (VGA) 470, filter 480for low-pass anti-aliasing, VGA 490, and is similarly coupled tophoneline connector 450 through electronic hybrid 440 andfilter/transformer/electronic protection circuit 445. Electronic hybrid440 and filter/transformer/electronic protection circuit 445 areconnected therebetween by a plurality of transmit and receive lines(e.g.,TX, RX1, RX2) 495. The operations of the analog front end are wellknown in the art.

[0038] Now turning to transmitter electrical characteristics, stationsat a minimum are capable of transmitting and receiving 2 MBaud modulatedframes in native V2 frame format. In a preferred embodiment stations arecapable of transmitting and receiving 2 Mbaud Compatibility V2 frameformat. Stations at a minimum are capable of transmitting allconstellations from 2 bits-per-Baud to 8 bits-per Baud (PE values 1-7)and receiving all constellations from 2 bits per Baud to 6 bits per Baud(PE values 1-5). The R.M.S. differential transmit voltage does notexceed −15 dBVrms in any 2-msec window between 0 and 30 MHz, measuredacross a 135-Ohm load between tip and ring for any payload encoding. Thepeak differential transmit voltage does not exceed 580 mVpeak for anypayload encoding at either 2 Mbaud or 4 M baud. Stations that are nottransmitting emit less than −65 dBVrms measured across a 100-Ohm loadbetween tip and ring. The electrical characteristics described below asto spectral mask apply to both the V2 native mode and the V2compatibility mode. The V2 metallic power spectral density (PSD) isconstrained by the upper bound depicted in the FIGS. 5a and 5 b with themeasurement made across a 100-Ohm load across tip and ring at thetransmitter wire interface. The mask applies to all payload encodings atboth 2 and 4 Mbaud. The resolution bandwidth used to make thismeasurement is 10 kHz for frequencies between 2.0 and 30.0 MHz and 3 kHzfor frequencies between 0.015 and 2.0 MHz. An averaging window of 213seconds used, and 1500-octet MTUs separated by an IFG duration ofsilence is assumed. A total of 50 kHz of possibly non-contiguous bandsmay exceed the limit line under 2.0 MHz, with no sub-band greater than20 dB above the limit line. A total of 100 kHz of possiblynon-contiguous bands may exceed the limit line between 13.0 and 30.0MHz, with no sub-band greater than 20 dB above the limit line. The 10 dBnotches at 4.0, 7.0 and 10.0 MHz are designed to reduce RFI egress inthe radio amateur bands. The mask is tested at PE values of 1 and 2 (2and 3 bits/symbol), as these payload encodings result in the maximumtransmitted power. The absolute power accuracy is +0/−2.5 dB relative to−7 dBm, integrated from 0 to 30 MHz. The passband ripple between 4.75and 6.25 MHz and between 8.0 and 9.25 MHz is less than 2.0 dB. Themagnitude of the V2 transmitter output is upper-bounded by the temporalmask shown in FIG. 6 for a compatibility mode pulse (the symbol responseof the 2.0 transmitter). The response is measured across a 100-Ohm loadbetween tip and ring at the transmitter's WIRE interface. Output beforet=0 and after t=5.0 microseconds is <0.032% of the peak amplitude. Thefirst compatibility mode pulse in the modified AID is exactly thetransmitter symbol response. The transmitter C-weighted output in theband extending from 200 Hz to 3000 Hz does not exceed 10 dBrnC whenterminated with a 600-Ohm resistive load. The transmitter emits no morethan −55 dBVrms across a 50-Ohm load between the center tap of a balunwith CMRR >60 dB and the transceiver ground in the band extending from0.1 MHz to 50 MHz. The transmitter clock frequency is accurate to within+/−100 ppm over all operating temperatures for the device. The minimumoperating temperature range for this characteristic is 0 to 70 degreesC. In general, a +/−50 ppm crystal meets this characteristic. The R.M.S.jitter of the transmitter clock is less than 70 psec, averaged over asliding 10-microsecond window. The differential noise output does notexceed −65 dBVrms across a 100-Ohm load, measured from 4 to 10 MHz withthe transmitter idle. There is no gain or phase imbalance in thetransmitter, except with respect to constellation scaling as describedabove.

[0039] Now turning to a comparable receiver's electricalcharacteristics, the receiver detects frames with peak voltage up to −6dBV across tip and ring at a frame error rate of no greater than 10⁻⁴with additive white Gaussian noise at a PSD of less than −140 dBm/Hz,measured at the receiver. The receiver detects 1518-octet frames framesencoded as 2 bits/symbol and 2 Mbaud with R.M.S. voltage as low as 2.5mV at no greater than 10⁻⁴ frame error rate. The R.M.S. voltage iscomputed only over time during which the transmitter is active. Thereceiver detects no more than 1 in 10⁴ 1518-octet, 2 bits/symbol, 2Msymbol/sec frames with R.M.S voltage less than 1.0 mV. Both criteriaassume additive white Gaussian noise at a PSD of less than −140 dBm/Hz,measured at the receiver, and assume a flat channel. The receiverdemodulates frames with payload encoded at 6 bits/symbol, 2 or 4 Mbaud(if implemented), and differential R.M.S voltage as low as 20 mV(measured over the header) at a frame error rate less than 10−4 underthe following conditions: (1) White Gaussian noise with PSD less than−130 dBm/Hz is added at the receiver, and (2) A single tone interfererwith any of the frequency band and input voltage combinations set forthin FIG. 7. The applied voltage is measured across tip and ring at theinput to the transceiver. The receiver demodulates frames with payloadencoded at 6 bits/symbol, 2 or 4 Mbaud (if implemented), anddifferential R.M.S voltage as low as 20 mV (measured over the header) ata frame error rate less than 10−4 under the following conditions: (1)White Gaussian noise with PSD less than −130 dBm/Hz is added at thereceiver, differential mode, and (2) A single-tone interferer, measuredbetween the center tap of a test transformer and ground at the input tothe transceiver, with any of the following frequency band and inputvoltage combinations set forth in FIG. 8. The common mode rejection ofthe test transformer used to insert the signal should exceed 60 dB to100 MHz.

[0040] The average return loss of the transceiver with respect to a100-Ohm resistive load exceeds 12 dB between 4.75 and 9.25 MHz. Thischaracteristic applies to the transceiver powered on or in low-powermode (transmitter powered off). The average return loss with respect toa 100-Ohm resistive load exceeds 6 dB between 4.75 and 9.25 MHz with thetransceiver removed from a source of power. The magnitude of the inputimpedance is >10 Ohms from 0-30 MHz and conforms to the lower-bound maskset forth in FIG. 9. This characteristic applies to the transceiverpowered on, in low-power mode (transmitter powered off), or removed froma source of power. FIG. 10 shows an example of the input impedance of acompliant device with a lower bound mask.

[0041] With regard to the receiver aspects in accordance with the PHYlayer protocol, reference in made to FIG. 11, wherein receiverfunctionality 900 is shown in block diagram form. Receiver functionality900 performs the reverse of that described above for transmitter 500,namely, upon receiving a signal from 2-4 wire hybrid and performingfront end processing as described in conjunction with FIG. 4b, thefollowing occurs: QAM/FDQAM Demodulator Gap Removal, ConsellationDecoding, De-scrambling and De-framing, as is well-known in the artgiven the above-defined transmitter functionality.

[0042] Referring back to the NID analog front end shown in FIG. 4b and aportion thereof shown in FIG. 12, in accordance with the presentinvention, a split winding transformer with turns ratios optimized formaximum transceiver signal to noise ratio is provided. More broadly, asplit winding transformer useful in a modem application is provided. Thetransmitter output signal level for typical modems is nominally fixedwithin some guardband of the FCC or other regulatory agency power limit.The signal level at the receiver input, however, is highly variabledepending on the channel attenuation in the path from a remotetransmitter. Consequently, the ideal line isolation transformer turnsratio from the transmitter output to the line of wt:1 is not optimal forthe receiver. At a modest additional cost of an additional transformerwinding, the turns ratios for the transmitter and receiver can be setindependently, while still allowing for hybrid echo cancellation. Sincethe receiver input signal will usually be less than the transmitteroutput signal, the optimal turns ratio is wt:1 from input to line, wherewr>wt. This step-up from the transmitted signal provides a “noiseless”gain that enhances the achievable receiver S/N ratio. The maximumseparation between wt and wr is limited by the reduction in couplingbetween transmit and receive windings that occurs for large differences.This introduces phase shift that compromises the effectiveness of commonecho cancellation schemes. Practical numbers for the wr:wt ratio arefrom 1 to 4. Prior art voiceband or ADSL modems do not take advantage ofthis technique. In the case of ADSL, the situation is particularlyegregious in that it is common to use step up transformers from modem toline side in order to boost the transmitted signals up to levelsrequired for long distance communication. This means there is actuallyattenuation of received signals. As can be seen in FIG. 12,filter/transformer/protection components 445, typically includingfilter/protection components 451 and transformer 453, is coupled in theTransmit and Receiver paths 495 and provides, for example, TX path 455and RX path 457 to be coupled to TIP and RING of Phoneline RJ11connector 450 through transformer 453. Wt:1 is transmit winding ratio;Wr:1 is the receive winding ratio; with the reference point, the twistedpair line, being 1. Transformer 453 couples the TIP line to the TXsignal path from electronic hybrid 440 via wt:1 windings. Transformer453 likewise couples the RING line to RX signal path via wr:1 windings.Therefore, in accordance with the present invention, a small signal onthe line being received can be stepped up, while on the transmit side onthe other hand, a stepping down can occur. Therefore in accordance withthe present invention a Wr of 2 provides a 1 to 2 step up, while on thetransmit side a Wt is 2/3 would, in essence provide a ratio of 3 to 1between the transmit and receive transformer windings. As set forth inFIG. 12 common core 459 is provided with three windings thereon, namelytip/ring winding 461 a, transmit side winding 461 b and receive sidewinding 461 c. The transformer is thereby optimizable to provide thebest signal to noise ratio for the transceiver.

[0043] Referring again back to FIG. 4B, in one embodiment implementingthe present invention a modem operating in half-duplex mode typicallyleaves the transmitter connected full-time to the hybrid and transformerdevices performing 4-wire to 2-wire conversion from modem to line, eventhough it is not active while a signal is being received. From a signalperspective, this has no consequence. However, the noise contributionfrom the transmitter output to the receiver input can be significant ina low-power signal environment. The addition of simple switch 435 (e.g.,a two transistor transmission gate in CMOS technology) between theoutput of the transmitter (e.g., filter 430) and hybrid 440 reducesnoise injected at the receiver input and therefore substantially improvereceiver S/N ratio. Activation of the switch can be incorporated into anautomatic gain control loop with the minimum gain control settingcausing the switch to turn off. Alternatively, a specific gain controlcode can be assigned to activate the switch, which can then be turnedoff (disabled) and on (enabled) in a directed manner.

[0044] As can be seen in the typical NID depicted in FIG. 4B, electronichybrid 440 feeds signal from the transmitter back into the receiver. VGA470 has two pairs of inputs, one fed back from the transmitter, theother a receive input from line 106. Any signal coming out of thetransmitter causes a self-echo path (e.g., through the transformerdepicted in FIG. 12) into the receiver that should be suppressed, suchthat the receiver does not get confused as to whether such self-echo isa signal coming from line 106. Noise also can get injected into thereceiver from the transmitting side, even during times when there is notransmitting, since the electronics components in the transmitting pathcan contribute noise, even when idle.

[0045] Therefore, in accordance with the present invention, when thetransmitter is not transmitting, transmit-off switch 435 provided in thetransmitting path, is switched off thereby blocking noise from gettinginjected back into the receive path which would deteriorate receiverperformance. As can be seen in FIG. 4B, in the preferred embodiment theswitch is located proximate to the end of the transmit path, i.e., justbefore combined electronic hybrid 440.

[0046] Those skilled in the art can appreciate that, while the presentinvention has been specifically described in conjunction with telephonelines in a home networking environment, other equivalent transmissionmedium could be used to implement the present invention. For example,the transmission medium for the frame-based communications network couldinclude power lines interconnecting transmitting and receiving stations.

What is claimed is:
 1. A method of optimizing signal transformation froma twisted pair transmission line to a combination transmitter andreceiver for a frame-based communications network, the transmitterhaving a transmit output pair port for transmitting signals onto theframe-based communications network over the twisted pair transmissionline, the receiver having a receive input pair port for receivingsignals from the frame-based communications network over the twistedpair transmission line, comprising: coupling a transformer between thetwisted pair transmission line and each of the transmit output pair portand the receive input pair port, the transformer having a coil acrossthe twisted pair, a transmit coil across the transmit output pair port,and a receive coil across the receive input pair port, wherein atransfer ratio between the transmit coil and the coil across the twistedpair is optimized for transmitting signals and a transfer ratio betweenthe receive coil and the coil across the twisted pair is optimized forreceiving signals.
 2. The method of claim 1, wherein the transfer ratiobetween the transmit coil and the coil across the twisted pair and thetransfer ratio between the receive coil and the coil across the twistedpair are optimized by optimizing transmit coil to coil across thetwisted pair turns ratio and receive coil to coil across the twistedpair turns ratio to maximize respective transmit path and receive pathsignal to noise ratios.
 3. The method of claim 1, wherein the twistedpair transmission line is a telephone line having a tip line and a ringline.
 4. The method of claim 1, wherein the transmit coil to coil acrossthe twisted pair turns ratio is designated wt:1 and the receive coil tocoil across the twisted pair turns ratio is designated wr:1, such that awr:wt ratio includes the range from 1 to
 4. 5. A transformer apparatusfor optimizing signal transformation from a twisted pair transmissionline to a combination transmitter and receiver for a frame-basedcommunications network, the transmitter having a transmit output pairport for transmitting signals onto the frame-based communicationsnetwork over the twisted pair transmission line, the receiver having areceive input pair port for receiving signals from the frame-basedcommunications network over the twisted pair transmission line,comprising: a plurality of transformer coils coupled between the twistedpair transmission line and each of the transmit output pair port and thereceive input pair port, the plurality of transformer coils including acoil across the twisted pair, a transmit coil across the transmit outputpair port, and a receive coil across the receive input pair port,wherein a transfer ratio between the transmit coil and the coil acrossthe twisted pair is optimized for transmitting signals and a transferratio between the receive coil and the coil across the twisted pair isoptimized for receiving signals.
 6. The transformer apparatus of claim5, wherein the transfer ratio between the transmit coil and the coilacross the twisted pair and the transfer ratio between the receive coiland the coil across the twisted pair are optimized by optimizingtransmit coil to coil across the twisted pair turns ratio and receivecoil to coil across the twisted pair turns ratio to maximize respectivetransmit path and receive path signal to noise ratios.
 7. Thetransformer apparatus of claim 5, wherein the twisted pair transmissionline is a telephone line having a tip line and a ring line.
 8. Thetransformer apparatus of claim 5, wherein the transmit coil to coilacross the twisted pair turns ratio is designated wt:1 and the receivecoil to coil across the twisted pair turns ratio is designated wr:1,such that a wr:wt ratio includes the range from 1 to 4.